The music and sound effects complement the already awesome visuals so much. And then the gameplay is interesting and unique as well. To quote a silly internet man "Space is so cool".
Edit: I just saw the special levels and I must say they are really stunning but 90% of players will probably not notice that the planets have those charging rings around them. Maybe that's intentional because I must say I felt proud of myself when I got to the result screen and saw the upsy downy path I took. There is so much to discover in this simple game.
Anyways, I have to stop rambling about how cool this game is so I can get to rating some more, but this is definitely my favorite so far.